Detection of function of implanted medical devices

ABSTRACT

A monitoring system and method for monitoring signals from an implantable medical device are disclosed. The monitoring system and method include a monitor configured to detect a radio frequency artifact from the signals of the implantable medical device and circuitry for processing the radio frequency artifact from the signals of the implantable medical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/685,810filed Oct. 15, 2003, now U.S. Pat. No. 7,315,760.

BACKGROUND

The present invention relates generally to the field of implantedmedical devices. More particularly, the invention relates to thedetection of pacing stimulus artifacts from implanted electronicpacemakers.

Detection of pacing stimulus artifacts from implanted electronicpacemakers in the body surface electrocardiogram (ECG) is sometimesdifficult due to sophisticated and more evolved generations of implanteddevices that stimulate the heart muscle. In addition, artifacts in thebody surface ECG have gotten smaller and/or more complicated in shapeand sequence. Further, pacemakers and cardio defibrillators emit theirown diagnostic signals which can sometimes confuse circuits.

Accordingly, there exists a need for improved detection of pacingstimulus artifacts from implanted pacemakers. Further, there exists aneed for improved detection/recognition of small pacer stimuli andimproved rejection of large pacer stimuli to increase reliability of thecritical functions of cardiac monitors and electrocardiographs. Further,there exists a need for the detection of artifacts regardless of whichmanufacturer has made the implanted pacer and regardless of whichversion of firmware or communications protocol is in the pacer.

SUMMARY

One embodiment of the invention relates to a monitoring systemcomprising an implantable medical device configured to be implanted in apatient to provide a stimulus to the patient and a monitor havingprocessing circuitry configured to detect a radio frequency artifactfrom the stimulus of the implantable medical device in order toeliminate an occurrence of falsely identifying voltage artifact as aheart beat.

Another embodiment of the invention relates to a monitoring systemcomprising a monitor configured to detect a radio frequency artifactfrom the signals of an implantable medical device and processingcircuitry configured to process the radio frequency artifact fromsignals of the implantable medical device in order to determine whereartifact occurs in an ECG and identify heart beats that are paced andheart beats that are not paced and occurrences of pacing that fail tostimulate a heart beat.

Another embodiment of the invention relates to a circuit for processingvoltage artifact from implantable pacemaker signals comprising a slewlimit circuit to limit pace artifact energy in the signals and a tunableband pass filter operable in parallel to the slew limit circuit andconfigured to isolate the voltage artifact from ambient noise and heartsignals.

Another embodiment of the invention relates to a method of monitoringsignals from an implantable medical device that provides a stimulus to apatient comprising detecting a radio frequency artifact from theimplantable medical device and processing the radio frequency artifactfrom the implantable medical device in order to determine where artifactoccurs in an ECG and identify heartbeats that are paced and heartbeatsthat are not paced and occurrences of pacing that fail to stimulate aheart beat.

Another embodiment of the invention relates to a system for monitoringsignals from an implantable medical device comprising a means fordetecting a radio frequency artifact from the implantable medical deviceand a means for processing the radio frequency artifact from theimplantable medical device in order to determine where artifact occursin an ECG and identify heart beats that are paced and heart beats thatare not paced and occurrences of pacing that fail to stimulate a heartbeat. The voltage artifact is excluded from heart rate determinations.

Another embodiment of the invention relates to a method for monitoring asignal from an implantable medical device comprising filtering voltagesamples from multiple channels of the signal, detecting at least one ofa leading edge and a trailing edge for a pulse, measuring width of apulse, measuring amplitude of the pulse, validating the pulse,classifying the pulse, and storing information about the pulse.

Another embodiment of the present invention relates to a system formonitoring a signal from an implantable medical device comprising ameans for filtering voltage samples from multiple channels of thesignal, a means for detecting at least one of a leading edge and atrailing edge for a sample, a means for measuring width of a pulse, ameans for measuring amplitude of the pulse, a means for validating thepulse, a means for classifying the pulse, and a means for storinginformation about the pulse.

Another embodiment of the present invention relates to a monitoringmethod comprising detecting a radio frequency artifact from signals ofan implantable pacemaker, processing the radio frequency artifact fromthe signals of the implantable pacemaker, determining where radiofrequency artifact occurs in an ECG, identifying heart beats that arepaced and heart beats that are not paced and occurrences of pacing thatfail to stimulate a heart beat, and excluding voltage artifact from aheart rate determination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an implantable medical device implanted ina patient in conjunction with an external monitor according to anexemplary embodiment.

FIG. 2 is a schematic representation of a software algorithm for pacedetection according to an exemplary embodiment.

FIG. 3 is a schematic representation of a monitoring system according toan exemplary embodiment.

FIG. 4 is a schematic representation of a monitoring system according toan exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, implantable medical device 10 provides an output 12to a patient 14. According to an exemplary embodiment, the outputincludes an intended stimulus provided to patient 14. According tovarious embodiments, output 12 may include electrical pacing signalssupplied by a pacemaker, electrical defibrillation signals, etc. Output12 may also include incidental output signals. For example, output 12may include a radio frequency signal (RF) 16.

A monitor 18 as shown in FIG. 1 is externally attached adjacent to orrelatively near the body of patient 14. According to a preferredembodiment, monitor 18 is attached in a position on the body of patient14 in relatively close proximity to output 12.

According to an exemplary embodiment, monitor 18 monitors signals fromimplantable medical device 10. According to a preferred embodiment,monitor 18 monitors and detects radio frequency (RF) artifact coincidentwith output 12 (e.g., pacemaker stimulus). Oftentimes, the current flowin the implanted pacing lead wires is short duration and generallyunshielded so that an RF artifact may be detectable by monitor 18comprising an antenna 19 placed on (or very near) a patient's bodysurface over the implanted pacemaker. The antenna 19 is preferablyintegrated into an existing lead wire such as an ECG electrode. Theantenna may be designed to be responsive to the electric field, themagnetic field, or both. According to various alternative embodiments,monitor 18 may also detect other types of unintentional output artifactsfrom implantable medical device 10 such as a magnetic field, anelectrical field, an acoustic sound, etc.

According to an exemplary embodiment, the improvements to the detectionof pacing stimulus artifacts from implanted electronic pacemakersincludes improved algorithmic processing 101. Referring to FIG. 2, thepace detection involves multiple steps including filtering at step 102,edge detection at step 104, width measurement at step 106, amplitudemeasurement at step 108, validating the pulse at step 110, classifyingthe pulse at step 112, and pace template storage at step 114. At step102 X, Y, and Z data is filtered. The voltage samples from each channelare filtered to accentuate the pacer pulse artifacts which are roughlyvery narrow rectangular pulses. A 250 Hz high pass will preserve thesharp edges needed to accurately measure pulse width and the amplitudeinformation of wider pacer pulses while rejecting many sources ofinterference like motion artifact and power line noise. This signal willoften have a value of about zero.

At step 104, edge detection occurs. Specifically, comparison of the mostrecent sample to the second most recent sample (e.g., about 50microseconds (μs) earlier) will generate a difference signal. When theabsolute value of this difference exceeds a threshold an edge isdetected. This threshold may be a fixed value such as 100 microvolts orit may be adjustable by the user. Preferably, it is adaptable by thesoftware to exceed by a factor such as 100 percent a noise levelestimated by the 90^(th) percentile of difference magnitudes in therecent one second of time. This is a leading edge if the current sampleis further from the zero value than the previous sample. By conventionthe edge at the start of a pulse is called the leading edge. The edge atthe end of a pulse is called the trailing edge.

At step 106, the width is measured. A second edge detected within ashort time, approximately less than 3 milliseconds (ms) of a leading(start) edge is considered a trailing (end) edge. The difference in timebetween leading and trailing edges is the width of the candidate pulse.If no trailing edge is detected within the approximately 3 ms period,the previous leading edge detection is discarded.

At step 108, the amplitude is measured. The sample having the value mostextreme (positive or negative) compared to zero between the leading andtrailing edges determines the amplitude of the pulse.

At step 110, the pulse is validated. A valid pacer pulse may haveamplitude in only one of the three channels, sometimes in only two, butusually in all three channels. The pulse is validated by measuring adistance between this pulse and several templates for pulses that arestored in a database. The timing of this pulse relative to recentlyvalidated pulses may also be checked against limits. If this pulse istoo soon in time as compared to previous pulses it may be invalidated.

For example, one sophisticated function of modern pacemakers is tosupply a pacing pulse to the patient at a lesser strength than previouspulses. This is a test to see if the pacemaker may reduce strength ofstimulus and extend battery life. However, there is a risk that the newstimulus pulse is not of sufficient strength to stimulate the heart andso a heart beat may not result. Accordingly, the pacemaker supplies asecond ‘safety’ pulse of the original higher strength shortly after thelower strength test pulse. This is done by the pacemaker even before theheart beat response is detected. A short while later the pacemakerdetermines from the timing of the heart beat response which of the firstor second pacing pulses was effective. Then the pacemaker can eitherretain the newer lower strength setting or reject that and continue withthe original strength pacing. This function may be performed only onceor a few times per day but it can result in two pacing pulses very nearin time to each other.

When three pacing pulses are observed in a very short period of time, itis likely that one of them is an artifact from something other than thepacer. Judging by the three axis (X,Y,Z) magnitudes and comparison tosaved templates it is possible to compute which of the three pulses ismost unlike any previous pulses. This pulse can then be rejected.

At step 112, the pulse is classified. If a pulse is validated it may beclassified as best matching one of the templates stored in the database.These templates may be classified as right atrial stimulus, left atrialstimulus, right ventricular stimulus, or left ventricular stimulus. Theclassification is based on the electrical axis of the stimulus(amplitude in X, Y, and Z channels) and on the relationship of thepulses to each other and to the atrial and ventricular response of theheart.

At step 114, the pace template is stored. These templates retaininformation relating to a history of recent pace spike observations. Theinformation consists primarily of amplitude in 3 channels (X, Y, Z) andthe pulse width. There may be four or more templates. As newly observedpulses are matched to the templates, the templates are updated. Pulseobservations that fail to match existing templates will force thecreation of new templates. The few initial pulses matching a newtemplate may be considered unusual and artificial and may beinvalidated. When a sufficient count of pulses have matched a newtemplate, the new template may be classified as non-artificial andfuture pulses matching it will be validated.

The cluster analysis is performed to group pacer pulse observations incategories useful to the cardiologist. Because a pacemaker, implanted orexternal, may be configured to stimulate possibly all four chambers ofthe heart, there may be up to four stimulus artifacts to detect during asingle heart beat. In practice, the number is more likely to be twopulses.

Stimulation of an atria and then a ventricle is known as AV pacing andhas been available for many years. Discrimination of these two pulsescould be done by timing alone since the atrial stimulus results in aP-wave in the electrocardiogram (ECG) and the ventricular stimulusresults in a QRS wave in the ECG and an atrial stimulus always precedesthe ventricular stimulus when both are used in the same heart beat.However, sometimes a pacemaker is configured to provide the atrial pulseonly when needed and the ventricular pulse only when needed. Such apacemaker for some heart beats may only supply one of the two pulses. Itbecomes more difficult then to determine which pulse was supplied. Inthe case where the heart may fail to respond to the pacemaker so thatthe stimulus pulse appears without the ECG P-wave or QRS wave responses,then it may be nearly impossible to determine which pacemaker stimuluswas supplied.

In very recent years pacemakers have been used to stimulate bothventricles for congestive heart failure patients. This is known asresynchronization therapy. In this case the two stimulus artifacts maybe much closer in time to each other then the atrial and ventricularpulses of the AV pacing described above.

Wherever a pacing lead is implanted it causes a stimulus current to flowin a certain path during the pulse resulting in a projection of thatvoltage onto the patient's body surface in a pattern where more voltageappears in some locations than others. The pattern will be different forthe different locations where the pacemaker stimulus contacts the heart.But for each location the pattern of subsequent stimuli will be similar.So this pattern can be used to group pacing stimuli into categoriesaccording to where the stimulus is given in the heart.

In the following table, a pulse was detected with a width of 100 μs andan amplitude in ECG lead X of 10 units and amplitude in lead Y of 30units and 20 units in lead Z. Another pulse was detected shortly afterwith amplitudes 50, 62, and 5 units in leads X, Y, and Z and a width of120 μs. After a longer period two more pulses were detected withcharacteristics reported in the 3^(rd) and 4^(th) rows of table 1.Cluster analysis enables all six pluses to be grouped into twocategories. Timing and direction rules are useful to identify theinterpretation of the pulses as atrial or ventricular, right or leftchambers. In other applications it is sufficient to classify some pulsesas non-pacer artifacts and others as recurring pacer pulses.

TABLE 1 Pace pulse observations, magnitude and width X Y Z width 10 3020 100 50 62 5 120 11 29 19 101 47 58 6 120 9 30 21 100 50 59 7 121

Referring to FIG. 3, ECG lead wire 150 connects an instrument to ECGelectrodes applied to a patient's body surface according to generallyacceptable standard placement locations used for electrocardiography.Multiple lead wires are actually used to convey multiple (preferably atleast three quasi orthogonal) channels of ECG voltages to the inputbuffer amplifiers 152. As used in this disclosure, quasi orthogonalgenerally means that the channels are minimally aligned with each otherso that the instrument may be sensitive to a pacing electrical vector inany direction of X, Y, Z space. According to an exemplary embodiment, asuitable choice of quasi orthogonal standard ECG channels may be leadsII (inferior), V1 (anterior), and V5 (lateral).

Alternative sets of quasi orthogonal ECG leads are desirable in theevent of loss of electrodes during monitoring or when different patientcables are used and fewer electrodes are available. A multiplexor in thecircuit can be configured as needed to connect the appropriate ECGchannels to the X, Y, and Z pace artifact processing channels. Table 2describes the preferred connections for four different patient cables.The patient cable is identified by the number of electrodes it connectsto the monitor. A usable alternative is a connection that might be usedwhen one of the preferred electrodes for that cable is no longeravailable. For example if the patient cable has at least six or tenelectrode connections, it is preferred to use ECG leads V5, II, and V1for X, Y, and Z inputs to the pace artifact processing. However, if theV1 electrode pops off accidentally it is a usable alternative toreconfigure the multiplexor so that the X channel is redirected to ECGlead V6 and the Z channel is redirected to ECG lead V3. This redirectionmay not be possible with the six electrode cable because electrodes V6and V3 are not present. However, for the six electrode cable two otherconfigurations are usable if those electrodes are still functioning.

TABLE 2 Preferred and optional lead sets for specified patient cablesPace Patient Cable Processing Electrode Channels Connection Count X Y Z3 5 6 10 I II III Preferred Usable Usable Usable I II V1 — PreferredUsable Usable V5 II V1 — — Preferred Preferred V6 II V3 — — — Usable

The input buffer amplifiers 152 are generally for increased inputimpedance of the instrument and to minimize leakage currents into thepatient. The three channels are further processed by filteringamplifiers 154 having sufficient high frequency response to preserve theimportant characteristics of the pacing artifacts. According to apreferred embodiment, amplifiers 154 have a frequency response in therange of about 0.05 to 10 KHz. The signal, analog up to this point, isdigitized by an analog to digital converter, shown as A/D converter 156at a high rate to sufficiently represent the short duration pacingartifacts. According to a preferred embodiment, A/D converter 156digitizes the signal at least at about 20,000 samples per second (sps),but preferably at about 100,000 sps.

The digital data stream is then passed to the software algorithm andutilized for two purposes, QRS detection and Pace detection. The QRSevent detector 200 may be of a type that generally uses strategies offurther bandpass filters, adapting thresholds, and correlation topreviously detected QRS shapes. These shapes are stored in the QRStemplate store 202. The QRS event classifier 204 uses rules todifferentiate normal QRS shapes from ventricular QRS shapes from pacedQRS shapes. These rules rely on the recognition of pacing artifacts atexpected time relationships to the QRS detections.

Returning to the ECG lead wires, an antenna or other suitable transducermay be combined with one special electrode or special lead wire to sensethe radio-frequency artifact (electric, magnetic, or both) resultingfrom the current flow within the patient's body during the pacemakerstimulus. This information is conveyed to a radio frequency detector 158which may be sampled by A/D converter 160. A/D converter 160 preferablyconverts the samples at a rate of about 500 samples per second. Thisdigital information is presented to the software algorithm, particularlythe pace event detector 206. Pace event detector 206 uses techniques(e.g., detect edge 104, measure width 106, etc. as shown in FIG. 2) torecognize a voltage pulse in the 20,000 to 100,000 samples per seconddata or the pulse of RF detected energy in the 500 samples per seconddata. These techniques utilize a pace template store 210 and aclustering algorithm within a pace event classifier 208 in order togroup recognized pacing artifacts into categories such as atrialstimulus, right ventricular stimulus, or left ventricular stimulus. Thedistribution of the magnitude of the pacing artifact in the three quasiorthogonal channels is a main discriminating characteristic. Withinformation about QRS classification and pace stimulus classification itis possible for the rhythm classifier 212 to determine whether thepatient is being paced, and if so, which of many types of pacingtherapies is currently being given.

The embodiment illustrated in FIG. 4 differs from the embodiment of FIG.3 in several ways. The example illustrated in FIG. 3 uses a highersample rate A/D conversion for the ECG voltage signal and more patternrecognition in the software than the first embodiment of FIG. 4. Theembodiment of FIG. 3 is designed for an instrument containing anauxiliary digital signal processor. FIG. 4 shows the same analogcircuitry for the radio frequency data (as shown in FIG. 3), but a splitpath for the voltage data and a lower sample rate ultimately going intothe software algorithm. FIG. 4 shows an embodiment where a high speedauxiliary digital signal processor is not utilized and all softwareexecutes on a more general purpose microprocessor. The split voltagesignal processing begins after being sent through the input bufferamplifiers 252. One path for the heart beat signal utilizes a slew limitcircuit 254 to exclude the pace artifact from this path. Slew limitcircuit 254 operates in a range of about 200 to about 400 mV/seconds,preferably at about 300 mV/second. ECG amplifiers 256 need less of ahigh frequency response because the pacing artifact has been removed.ECG amplifiers 256 operate in a range of about 0.025 to about 300 Hz,preferably from about 0.05 to about 150 Hz. An A/D converter 258 is usedfor digitizing a diagnostic quality ECG signal without pacer artifacts.A/D converter 258 operates in a range of about 400 to about 600 samplesper second, preferably about 500 samples per second (sps).

The second path of the voltage signal processing begins with specializedPACE amplifiers 260 for the PACE signal (e.g., PACE amplifiers Gain,center frequency, band width, etc.). These are analog circuitamplifiers, again in three quasi orthogonal channels (although only onechannel is depicted in the figure). Using generally known techniquesthese amplifiers may be controlled (dotted line) to have adjustablegain, center frequency (FC) and band width (BW). In this way theamplifiers can be adapted over time to optimize the filtering for thepacer stimulus. The output of the programmable filter amplifier is thensent into the peak stretcher 262 (shown as peak stretcher logarithmic)which may preferably have a logarithmic amplitude response. Preciserepresentation of the pacer stimulus magnitude is less important thanits distribution in the three quasi orthogonal channels. Because thepossible stimulus magnitudes span a very wide dynamic range (very smallartifacts to very large artifacts) it is useful to contract the dynamicrange by a logarithm function so that the subsequent A/D converter 258(at 500 sps) may not require such a wide code word (e.g., a fewer numberof bits in the digital code). This logarithm function means that smallartifacts will be adequately represented in the three channels whilevery large artifacts can also be adequately represented. The edge timercircuit 266, by processing in the analog domain, may have time precisionmuch increased compared to what the software algorithm could otherwisedetermine from the 500 sps data which has 2 millisecond gaps betweensamples.

Returning to the ECG lead wires, an antenna or other suitable transducermay be combined with one special electrode or special lead wire to sensethe radio-frequency artifact resulting from the current flow within thepatient's body during the pacemaker stimulus. This information isconveyed to a radio frequency detector 268 which may be sampled by A/Dconverter 258. A/D converter 258 preferably converts the samples at arate of about 500 samples per second. This digital information ispresented to the software algorithm, particularly the pace eventdetector 206. The algorithm may be the same or similar to that shown anddescribed in FIG. 3.

It is important to note that the above-described preferred embodimentsare illustrative only. Although the invention has been described inconjunction with specific embodiments thereof, those skilled in the artwill appreciate that numerous modifications are possible withoutmaterially departing from the novel teachings and advantages of thesubject matter described herein. Accordingly, these and all other suchmodifications are intended to be included within the scope of thepresent invention as defined in the appended claims. The order orsequence of any process or method steps may be varied or re-sequencedaccording to alternative embodiments. In the claims, anymeans-plus-function clause is intended to cover the structures describedherein as performing the recited function and not only structuralequivalents but also equivalent structures. Other substitutions,modifications, changes and omissions may be made in the design,operating conditions and arrangements of the preferred and otherexemplary embodiments without departing from the spirit of the presentinvention.

1. A method of monitoring signals form an implantable medical devicethat provides a stimulus to a patient comprising: detecting a radiofrequency artifact from implantable medical device; and processing theradio frequency artifact from the implantable medical device in order todetermine where artifact occurs in an ECG and identify heartbeats thatare paced and heartbeats that are paced and heartbeats that are notpaced and occurrences of pacing that fail to stimulate a heart beat. 2.The method of claim 1 further comprising excluding artifact from heartrate determinations.
 3. The method of claim 1 wherein detecting theradio frequency artifact comprises using a monitor having an antennathat receives the signals.
 4. The method of claim 1 wherein processingthe radio frequency artifact comprises using circuitry capable ofmonitoring multiple channels of the signals.
 5. The method of claim 4wherein processing the voltage artifact further comprises using a slewlimit to limit pace artifact energy in the signals.
 6. The method ofclaim 4 wherein processing the voltage artifact further comprises usinga tunable band pass filter to isolate the radio frequency artifact fromambient noise.
 7. A monitoring method comprising: detecting a radiofrequency artifact from signals of an implantable pacemaker; processingthe radio frequency artifact from the signals of the implantablepacemaker; determining where radio frequency artifact occurs in an ECG;identifying heart beats that are paced and heart beats that are notpaced and occurrences of pacing that fail to stimulate a heart beat;excluding voltage artifact from a heart rate determination.
 8. Themethod of claim 7 further comprising classifying pacing stimuliaccording to heart chamber location.
 9. The method of claim 7 whereindetecting the radio frequency artifact comprises using a monitor havingan antenna that receives the signals.
 10. The method of claim 7 whereinprocessing the radio frequency artifact comprises using circuitrycapable of monitoring multiple channels of the signals.
 11. The methodof claim 7 wherein processing the voltage artifact further comprisesusing a slew limit to limit pace artifact energy in the signals.
 12. Themethod of claim 10 wherein processing the voltage artifact furthercomprises using a tunable band pass filter to isolate the radiofrequency artifact from ambient noise.
 13. A method of monitoringsignals form an implantable medical device that provides a stimulus to apatient comprising: detecting a radio frequency artifact fromimplantable medical device; processing the radio frequency artifact fromthe implantable medical device in order to determine where artifactoccurs in an ECG and identify heartbeats that are paced and heartbeatsthat are paced and heartbeats that are not paced and occurrences ofpacing that fail to stimulate a heart beat; classifying pacing stimuliaccording to heart chamber location; and excluding artifact from heartrate determinations.
 14. The method of claim 13 wherein detecting theradio frequency artifact comprises using a monitor having an antennathat receives the signals.
 15. The method of claim 13 wherein processingthe radio frequency artifact comprises using circuitry capable ofmonitoring multiple channels of the signals.
 16. The method of claim 15wherein processing the voltage artifact further comprises using a slewlimit to limit pace artifact energy in the signals.
 17. The method ofclaim 15 wherein processing the voltage artifact further comprises usinga tunable band pass filter to isolate the radio frequency artifact fromambient noise.